1. Technical Field
The present disclosure relates to a wavelength converting apparatus used in a laser apparatus.
2. Related Art
Along with development of micronizing and high integration of semiconductor integrated circuits, an improvement in resolution is required in semiconductor exposure devices. Hereinafter, a semiconductor exposure device will be simply referred to as an “exposure device”. Accordingly, a wavelength of light output from an exposure light source is getting shortened. As an exposure light source, a gas laser is used, instead of a conventional mercury lamp. At present, as for laser apparatuses for exposure, a KrF excimer laser that outputs ultraviolet light having a wavelength of 248 nm, and an ArF excimer laser that outputs ultraviolet light having a wavelength of 193.4 nm are used.
Currently, as an exposure technology, immersion exposure has been put into practice. In the immersion exposure, a space between a projection lens on the exposure device side and a wafer is filled with liquid, whereby the refractive index of the space is changed. Thereby, an apparent wavelength of the light source for exposure is shortened. In the case where immersion exposure is performed with use of an ArF excimer laser as a light source for exposure, a wafer is irradiated with ultraviolet light having a wavelength of 134 nm in the water. This technology is called ArF immersion exposure. ArF immersion exposure is also referred to as ArF immersion lithography.
The spectral linewidth in natural oscillation in KrF and ArF excimer lasers is wide, approximately ranging from 350 pm to 400 pm. This causes chromatic aberration of laser light (ultraviolet light) reduced and projected on the wafer by the projection lens on the exposure device side. Thereby, the resolution is lowered. As such, it is necessary to narrow the spectral linewidth of laser light output from a gas laser to a degree in which chromatic aberration can be disregarded. Accordingly, in a laser resonator of a gas laser, a line narrowing module having a line narrowing element is provided. By the line narrowing module, narrowing of the spectral linewidth is realized. The line narrowing element may be an etalon, a grating, or the like. A laser apparatus in which the spectral linewidth is narrowed as described above is referred to as a line narrowed laser apparatus.
Further, as a laser apparatus for an exposure device, a solid-state laser apparatus may be used from a viewpoint of energy saving. A solid-state laser apparatus includes a semiconductor laser, a nonlinear crystal, and the like.